This invention relates to a method of dispersing additives normally solid at room temperature in finely divided solid polymer.
Conventional methods of dispersing additives such as stabilizers and pigments in solid polymers include melt blending the additives and the polymer on a roll mill, in a compounding extruder, or in an intensive mixer such as a Banbury mixer. Besides being a costly operation, the mechanical working necessary to effect adequate dispersion subjects the polymer to high shear at elevated temperatures where the polymer is molten and such working tends to degrade the polymer, particularly polymer which does not yet have a stabilizer well dispersed through it. Many efforts have been made to circumvent these shortcomings, for instance, by dry blending the additives with the polymer. However, when rather high concentrations of additives are blended with the solid polymer in making masterbatches for letting down into virgin polymer a sticky and gummy mixture often is formed. This sticky mixture accumulates on the blenders and other equipment. At times such mixture has restricted and completely plugged equipment thus shutting down a plant until the material causing the problem is removed. In addition such material often is difficult to disperse resulting in uneven distribution of the additives which is almost always unacceptable. In the case of antioxidants the portions of the polymer receiving less than the desired amount of antioxidant tend to degrade rapidly in use or even during fabrication. Conversely when antioxidant which has plated out onto the blender flakes off, polymer results with an excessive amount of antioxidant present. Depending on the antioxidant, this may result in discoloration, odor, or other deleterious effects.
The undesirable results from blending normally solid additives into polymer can be alleviated to some extent by first blending the polymer with some liquid dispersing agent which acts to stick the additive to the polymer. This is an inadequate solution since it necessitates an extra step and introduces an unnecessary and in many cases undesirable extra ingredient into the blend; nevertheless it is used on a commercial scale at the present time thus pointing up the need for a simple solution to the problem of dispersing normally solid additives in a polymer.
It is an object of this invention to disperse normally solid additives in solid polymer.
It is a further object of this invention to obtain excellent dispersion of additives in polymers without either the use of costly and possibly degrading melt blending or the introduction of unnecessary foreign materials such as dispersants.
It is still a further object of this invention to prepare a masterbatch of normally solid additives in polymer without either the use of costly and possibly degrading melt blending or the introduction of unnecessary foreign materials such as dispersants.